Litmus paper
Litmus paper is a filter paper which has been treated with a natural water-soluble dye obtained from lichens.[1]
Litmus paper is the most recognized member of chemical indicators. Like most pH paper, litmus changes color when exposed to an acidic or alkaline solution. The simple pH scale ranges from 0-14 with 0 being the most acidic, 7 being neutral, and 14 being the most basic or alkaline. Litmus paper is commonly used in educational science classes.[2]
History
The term litmus comes from an Old Norse word meaning "to dye or color." This is fitting since the lichens used to make litmus have also been used to dye cloth for hundreds of years. Very little information is available about the beginnings of litmus. There is some data that suggest that litmus paper was developed by J.L. Gay-Lussac, a French chemist during the early 1800s. [2]
Composition of litmus paper
| Litmus (pH indicator) | ||
| below pH 4.0 | above pH 8.5 | |
| 4.0 | ⇌ | 8.5 |
Litmus is actually a dye made from naturally occurring substances. Raw materials using for making litmus include wood cellulose, lichens and adhering compounds. The wood being used for making pulp is treated with solvents to remove lignin or resinous material. Pretreatment of lichens with a solution of urine, potash and lime is followed by addition of Plaster of Paris. Moulding is done to get small cakes. Paper treated with litmus is known as litmus paper or litmus strips.The reason litmus paper shows color change is the infusion of lichens into litmus paper. [3]
Usage
Red litmus paper is used to detect basicity. It turns to blue under basic conditions and is unchanged under acidic conditions. Red litmus paper is prepared by the treatment of plain filter paper with a red dye. This red dye is pre-treated with little sulphuric acid and dried in air. [3]
Blue litmus paper is used to identify acids like sulphuric acid or nitric acid. It changes color from blue to red to indicate the presence of acids. It is made by the treatment of plain paper with lichen blue dye. [3]
-
Blue litmus paper reacts with Sodium hydroxide -
Red litmus paper reacts with hydrochloric acid
If both the red and blue papers do not change color, the test solution is approximately neutral in pH.
Gallery
References
- ^ Anne, Marie. "Litmus paper definition". about education. Retrieved 16 February 2016.
- ^ a b Delisle, Sandy. "Litmus Paper". Made How. Retrieved 18 February 2016.
- ^ a b c "What is a Litmus Test". Innovate Us. Retrieved 20 February 2016.
